Scientific Forecaster Discussion

Synopsis... a warm front will lift through the area overnight...followed by a Arctic cold front during the day Wednesday. The front will push to our south into Thursday while low pressure develops and moves along the boundary. High pressure will return to the middle Atlantic region for Friday and Friday night. A frontal passage is expected Saturday into Saturday night...followed by high pressure again Sunday into Sunday night. Another frontal passage is expected Monday into Monday night.

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Near term /until 6 am Wednesday morning/... the center of high pressure has shifted offshore this afternoon. Meanwhile, a trough of low pressure was lifting northeastward across the upper Midwest. Southerly flow in between these two systems will continue to strengthen over the eastern Seaboard. Warm and moist air transported by these winds will overrun the retreating cold air that was previously over our area.

The leading edge of isentropic lift and low-level convergence matched up well with the band of precipitation that is currently moving into the Delaware-Maryland-Virginia and eastern PA. The leading precipitation will continue to move northeastward into the rest of the County Warning Area just in time for the start of the afternoon commute. Snow had already changed over to sleet at ridge-ptw-ilg and liquid (rain/freezing rain) over south-central PA and along the Chesapeake Bay as of 230 PM. The trend was to speed up the transition to sleet and rain/freezing rain by a couple of hours from earlier forecasts based on the latest observation and mesoscale model guidance. Dewpoints are low enough that at locations where temperatures are just above freezing prior to the onset will wet-bulb down to at or just below freezing once precipitation starts. Expanded the Winter Weather Advisory southward into central Delaware-Maryland-Virginia and far southern New Jersey. A brief period of freezing rain will quickly develop over these areas since the faster changeover occurred before the surface temperatures were able to respond to the warming southerly winds. The timing of the changeover to freezing rain looks to coincide with the start of the evening commute for the I-95 corridor. Even with minimal icing, travel will be adversely impacted as untreated roads and sidewalks become slippery, especially considering the timing.

Unlike the event last Sunday where there was not much wind and warm air advection in the boundary layer, the strong southerly winds aloft will be able to have more of an influence in the boundary layer, which will be much more effective at scouring out the cold air; so the window of freezing rain will be brief. The one exception may be the I-78 corridor northward where the terrain could trap the cold air in the typical sheltered valleys. We are expecting the freezing rain to persist into the overnight in these northern areas and locally higher amounts of icing between one and two tenths of an inch are possible.

Much of the coastal plain is forecast to rise into the 40s tonight with a southeast to southwest wind. Readings are expected to be mainly in the 30s in areas to the northwest of the Interstate 95 corridor.

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Short term /6 am Wednesday morning through 6 PM Wednesday/... Winter Weather Advisory continues into the first part of Wednesday morning for the Poconos and far northwest NJ, where pockets of sub-freezing temperatures and freezing rain will persist. Otherwise, Wednesday will be a relatively quiet period in between two winter events.

A cold front will approach from the northwest in the morning and slowly sag southeastward throughout the day. Expect period of rain throughout the day for much of the area. The coverage of rain will be highest near this frontal boundary. Quantitative precipitation forecast amounts will be light- less than a half inch. Leaned toward the cooler met guidance for temperatures given the persistent clouds and precipitation over the area, as well as the end to the warm air advection regime with winds veering out of the south and southwest. Highs Wednesday range from the middle 30s in the Poconos to the upper 40s in southern Delaware.

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Long term /Wednesday night through Tuesday/... shortwave energy rounding the base of a middle-level trough situated over Great Lakes will gradually veer large scale flow to the southwest over the mid-Atlantic. This will slow the advance of an Arctic front moving through the area on Wednesday and it will briefly become stationary to our south on Thursday. Meanwhile...closed middle-level low ejects northeastward out of Baja California California...accompained by several southern stream shortwave disturbances that will lead to low pressure development along the now stationary front to our south. This will promote a robust overrunning regime over the mid-atl... fed by Baja California/greater Mexico moisture and supported by the lf quadrant of a vigorous 250 hpa jet.

The operational models and their respective ensembles have signaled this scenario for over a week now...and finally appear much closer to a consenus on the low-level thermal profile...but differences remain in their depiction of the quantitative precipitation forecast bullseye and amounts. At this time...we favor the NAM/European model (ecmwf) quantitative precipitation forecast...and feel the GFS is overdone. This would place the maximum quantitative precipitation forecast bullseye in the 00z Wednesday through 00z Thursday time frame along the I-95 corridor. As colder air works in from the northwest...we expect rain to gradually change to snow from north to south Wednesday night. Keep in mind there is still some disagreement on how quickly the low-level thermal profile cools...as slpr fields indicate the region is in a saddle...which is supportive of a slow cold frontal passage. At this time...we expect the rain to changeover to snow in the northwest around 00z...and perhaps as late as 09z in the lower Delaware-Maryland-Virginia.

As far as ptype and amounts...we're expecting 6 to 8 inches of snow in the I-95 corridor with 4 to 6 inches north and south of this area. If a more robust warm nose works in aloft...as suggested by the NAM...there would be more sleet which would cut down on the snowfall amounts. On the other hand...although Omega in the dendritic snow growth zone is not robust...fgen is indicated in the middle-levels and this could boost snowfall totals. Finally...it is questionable how much dry air and/or downsloping due a northerly surface flow will cut down on quantitative precipitation forecast across the northern part of the County Warning Area. Further to the south over the Delaware-Maryland-Virginia...snowfall totals could be lower due to a warmer thermal profile...implying a longer transition from rain to snow.

With this package we have issued a Winter Storm Watch for the entire County Warning Area from Wednesday night through Thursday evening...with the greatest uncertainty in meeting snowfall criteria north of the I-78 corridor and over far southeast New Jersey and the southern Delaware-Maryland-Virginia. We expect the Thursday morning rush hour to be particularly impacted...especially in the I-95 corridor...where snow could fall moderate at times.

Snow is expected to end Thursday evening...as the middle-level trough approaches from the west...and pushes the system offshore. Much colder air will filter in Thursday night with min temperatures around 20 degrees below normal. Otherwise...generally fair weather is expected through the weekend... with a weak cold frontal passage likely later Sat or early Sunday. The front has limited moisture at this time...so the forecast is kept dry. Temperatures may actual approach seasonable levels on Sunday. Early next week...there is a lot of uncertainty with regard to the evolution of both northern and southern stream energy...and the degree of phasing that takes place between the two over the eastern Continental U.S.. for now we kept this part of the forecast dry as well.

VFR conditions will deteriorate to MVFR as precipitation starts to overspread the region. Thermal profiles show a warm layer deepening above the surface which will cause sleet to fall, mixing with snow early, with and than a quick transition to freezing rain at the terminals late this afternoon. All terminals are expected to change over to plain rain by around 03-05z except maybe Abe for a few hours into the overnight. Conditions will remain MVFR/IFR through Wednesday.

Winds out of the south this afternoon will continue to gradually veer towards the southwest tonight. In addition, winds around 2000ft above the surface are around 50 knots while winds at the surface are less than 10 knots. Low level wind sheer will be an issue from this evening through late. Wind sheer will lessen by early Wednesday and winds will veer to the west and gradually becoming northwest Wednesday morning.

Outlook... Wednesday night and Thursday...rain will gradually changeover to sleet... then snow across the terminals from north to south. The changeover is expected to occur at Abe and ridge between around ooz Thursday...at ttn, pne, phl, and ilg around 03z Thursday...and miv and Acy around 06z Thursday. There is still some uncertainty as to what time the change takes place. Expect MVFR conditions with IFR possible in snow and low ceilings...particularly at ttn, pne, phl, and ilg.

Thursday night and Friday...snow will gradually end from west to east across the terminals Thursday evening. Expect MVFR/IFR conditions to improve to VFR by Thursday night and remain VFR through Friday.

Saturday and Sunday...MVFR possible in low clouds and rain showers/snsh with the passage of a cold front late Sat into early Sunday... otherwise VFR conditions expected.

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Marine... southerly winds will increase this evening. Although storm-force winds will be located only 1,000 feet above ground level and gale-force winds 200 feet agl, a strong inversion will not be favorable for these winds to mix down to the surface. However, the pressure gradient tightens enough for sustained winds to approach Small Craft Advisory criteria tonight. Seas are also forecast to build to 4-5 feet overnight. Therefore, a Small Craft Advisory was issued for the coastal waters.

The winds turn more westerly on Wednesday and decrease just ahead of a cold front. Do not expect Small Craft Advisory gusts. However, elevated seas near 5 feet may linger into the morning. Confidence wasn't high enough to extend the advisory at this point.

Friday afternoon through Sunday...winds and seas are expected to remain below Small Craft Advisory thresholds.

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Hydrology...with last night's solutions looking a little colder and with less liquid before the changover to snow on Thursday, we became much less concerned about any flooding on Wednesday into Wednesday night.

This morning's runs came in wetter Wednesday afternoon and Wednesday night, especially south of philly. The GFS, for instance, has 0.75 to about 1.00 of liquid across the Delaware-Maryland-Virginia and southern NJ, south of the i295 corridor. We're once again taking notice.

A changeover to sleet or snow isn't expected to occur until Thursday morning.

While there isn't much of a snowpack across the southern half of our cwa, the ground is frozen, and any runoff will be very efficient.

If the 0.75 to 1.00 materializes, we feel nuisance poor drainage and low lying flooding will be the most likely outcome. Modeled solutions suggest we would need about 2.50 inches of liquid, either in the form of all rain or rain and melting snow to have our southern creeks and streams leave their banks.

With more snow or sleet expected across the northern half of the cwa, we're not concerned about flooding.

With the rainfall, the flow on area creeks and streams will increase. This could start to move ice, and in a worst case scenario, create restrictions or jams.

Continue to monitor the forecast. Keep in mind that snowmelt and the warmer temperatures expected tomorrow, by themselves, would not cause flooding. The amount of rain that we receive will be the determining factor.